1. Field of the Invention.
The present invention relates generally to chemical cleaning compositions and methods, and more particularly, but not by way of limitation, to compositions and methods for removing iron containing deposits from metal surfaces
2. Description of the Prior Art.
The operation of various equipment such as feed water heaters, steam boilers, equipment associated with service water systems and other equipment in which water is circulated is often hindered by the formation of water insoluble deposits or scale on the interior surfaces thereof. The formation of such deposits can interfere with proper heat transfer, decrease the capacity of the flow passages in the equipment and lead to leaks and ruptures which necessitate undesirable down time and maintenance costs.
Many cleaning methods and solvents have been developed for removing various types of deposits from the interior metal surfaces of equipment. Typical solvents include acids such as hydrochloric acid and nitric acid, or ammonia or amine salts of organic chelating acids such as citric acid or ethylenediaminetetraacetic acid (EDTA). The solvents are circulated through the equipment under conditions and for a period of time sufficient to remove the deposits. This allows the deposits to be effectively removed without the expense and time required to dismantle the equipment.
In any cleaning process, it is desirable to effectively remove the deposits without causing corrosion or other damage to the metal surfaces of the equipment being cleaned. In order to save time and to minimize the amount of waste requiring disposal, it is desirable to remove the deposits and passivate the metal surfaces with only one solvent fill.
Severe corrosion and other damage can result to the metal surfaces forming the equipment being cleaned if the wrong solvent is used. For example, nitric acid can cause severe corrosion damage to carbon steels, copper, and copper bearing alloys such as brass and bronze. Chloride ions from hydrochloric acid can cause stress corrosion cracking to occur in stainless steels. Copper and copper bearing alloys such as brass and bronze are subject to failure by stress corrosion cracking when exposed to ammonia or amines. Thus, when prescribing a solvent, the type of metal forming the equipment as well as the type of deposits formed thereon must be carefully considered.
Prescribing a solvent is not difficult if the equipment to be cleaned is constructed of only one type of metal. A great deal of equipment, however, is constructed of many different types of metals. For example, equipment associated with service water systems is commonly constructed of stainless steel, carbon steel, copper, brass, and other alloys. Such equipment often becomes scaled with iron oxides and other deposits having densities of 10,000 g/m.sup.2 (929 g/ft..sup.2) or more. It is difficult to prescribe a solvent that will effectively remove these deposits without causing corrosion or other damage to some of the metals forming the equipment. Many service water systems have to be dismantled so that their different metal surfaces can be cleaned separately.
Many of the cleaning methods and solvents developed heretofore are very capable of removing high density iron containing deposits from carbon steels and stainless steels. In U.S. Pat. No. 3,072,502, a process is disclosed in which copper and iron oxide scale is removed from metal surfaces with a chelating solvent containing citric acid. The solvent is adjusted to the required pH by a nitrogen containing base such as triethanolamine or ammonia. In U.S. Pat. No. 3,438,811, a process is disclosed in which copper and copper containing scale is removed from metal surfaces with a chelating solvent containing a polycarboxylic acid, e.g., ethylenediaminetetraacetic acid (EDTA), and/or one or more amine or ammoniated salts thereof. Unfortunately, the solvents used in these processes contain ammonia and/or amines. As a result, they are not suitable for cleaning copper and copper bearing alloys such as brass and bronze.
Prior to the present invention, it was generally accepted by those skilled in the art that a nitrogen containing base such as ammonia or some type of amine must be present in solvents containing chelating acids such as citric acid or ethylenediaminetetraacetic acid (EDTA) for the solvents to effectively dissolve iron. It was thought that ferrous or ferric ammonium salts of the acids were formed when iron containing deposits were dissolved in the solvents. It was believed that the ferrous or ferric ammonium salts of the acids were necessary to prevent precipitation from occurring.
By the present invention, it has been discovered that it is ferrous or ferric salts, not ferrous or ferric ammonium salts, that are formed when iron containing deposits are dissolved in solvents containing chelating acids such as citric acid or ethylenediaminetetraacetic acid (EDTA) and a nitrogen containing base. For example, when iron containing deposits are dissolved by a solvent containing citric acid and ammonia, ferrous citrate, not ferrous ammonium citrate, is formed. Ferrous salts of chelating acids such as citric acid and ethylenediaminetetraacetic acid (EDTA) are more soluble at moderately alkaline pH levels than at acidic pH levels. It is the pH of the solvents, not the presence of a nitrogen base in the solvents, that prevents precipitation from occuring and results in the ability of the solvents to effectively dissolve iron containing deposits.
Inasmuch as an amine or ammonia is not necessary for the solvents to be effective, any base capable of raising the pH to the required level can be used. If bases that do not contain an amine or ammonia are used to raise the pH, solvents containing chelating acids such as citric acid and ethylenediaminetetraacetic acid (EDTA) can be used to remove iron containing deposits from all types of metals, including copper and copper bearing alloys such as brass and bronze. Such solvents can be used to clean equipment constructed of dissimilar metals without causing corrosion or other damage thereto.